Phase locked loops are well-known in the art. A long standing trade-off in the design of such loops has been bandwidth (signal to noise ratio) versus stability versus slew rate. On the one hand, it is desirable to have a wide bandwidth in order to ensure capturing of the incoming signal. On the other hand, a narrow bandwidth is desirable to reduce the amount of noise, i.e. provide a high signal to noise ratio. In order to ensure capturing of an incoming signal, a narrow bandwidth has to slew fast enough to adjust to variations in the incoming signal frequency. However, loops with a fast slew rate suffer from instability and may drift too much, i.e. out of the range of the incoming signal and hence be unable to capture such signal.
Loops with a high Q are very stable, however too high a Q is not acceptable because the loop must still be able to track jitter. The shape of an incoming data pulse may have a series of superimposed small ripples and peaks. A square wave pulse shaper in a remote repeater or at the signal input responding to some level of the leading or trailing slope of the pulse may be triggered slightly earlier or later if a small peak or valley occurs at or around the threshold level. This causes a fuzzy area, or jitter, at the leading and trailing straight edges of the generated square wave (Input Data). The loop should be able to adjust to or track this jitter.
A standard LC oscillator affords a high slew rate, but suffers from instability, particularly drifting with component aging and temperature. A crystal oscillator is extremely stable, however it will not slew fast enough to track jitter because of its large "inertia" (Q).
The present invention provides the aforenoted previously incompatible results, and does so in a particularly simple and efficient manner.